In the production of thermoplastic plastics materials, it is often necessary to remove residual monomers from the starting plastics material. For example, if a packaging film for foodstuffs is to be produced by extrusion, the residual monomers may be harmful to health. For medical reasons, therefore, the residual monomer content is not permitted to exceed a predetermined limit value which is laid down in regulations governing foodstuffs. If the film is to be made from extruded polystyrene, the polystyrene often contains up to 100,000 parts per million of monomeric styrene and a highly effective degassing arrangement is necessary in the extruder in order for the amount of the residual monomers present in the final product to be reduced to below the limit amount which is of the order of 1000 parts per million.
In the degassing of such plastics materials in a single-screw extruder, it has long been known to provide an injection location for a stripping agent, with which the molten mass of plastics material is mixed in the extruder. In a degassing section disposed downstream of the injection location, this stripping agent, which is generally water, ensures that the molten mass foams if the extrusion pressure reduces. The increase in the free surface area of the molten mass, caused by such foaming effectively degasses the molten mass which is then suitable for a wide range of applications.
The proportion of residual monomers present in the product produced in a single-screw extruder is, however, still not sufficiently low for the product to be utilisable in many fields of application, particularly those where the plastics material product is intended to come into direct contact with, for example, foodstuffs.
In German Offenlegungsschrift No. 26 25 609, there is disclosed a twin-screw extruder for the degassing of molten masses containing residual monomers. Particular problems arise, however, with a twin-screw extruder because of its structural features. In particular, the nip region of the extruder, that is to say, the region in which the cross-section of the processing chamber of the extruder is constricted, prevents the desired pressure in the molten mass, which is necessary for optimum mixing of the molten mass with the stripping agent, from being achieved.
To solve this problem, this prior document proposes the provision of adjusting and locking rings upstream and downstream respectively of the injection location for the stripping agent. These rings are mounted on the extruder screws and rotate therewith. In such a case, the diameter of the upstream adjusting ring should substantially be equal to the external diameter of the extruder screw with which it is associated. However, the diameter of the locking ring, which is disposed downstream of the injection location, should be somewhat greater than that of the adjusting ring.
The nip region can be unilaterally constricted by means of a substantially V-shaped operating barrel which protrudes into the processing chamber and which is adapted to the circular shape of the adjusting rings. In consequence, the quantity of material conveyed between the internal wall of the barrel and the adjusting rings can be regulated to a certain extent, as can the pressure in the extruder. A stream of molten material, which passes in such a manner into the injection region between the adjusting and locking rings, is brought into contact with the injected water by means of kneading means on the extruder screws. The water evaporates upon contact with the molten mass and this causes foaming in the adjacent degassing section of the extruder.
The degassing efficiency of such an extruder is undoubtedly better than that of a twin-screw extruder without any means for controlling the stream of molten mass passing into the kneading section. However, even such an arrangement suffers from the disadvantage that the injected water evaporates immediately upon contact with the molten material and, in consequence, only a very poor mixing of water and the molten material occurs.